1. Field of the Invention
This invention relates to a process for purifying crude isoprene. More particularly, it is concerned with a process for the removal of polymerization inhibiting materials such as acetylenes, sulfur compounds and the like contained in crude isoprene which is separated by known procedures such as extractive distillation or liquid-liquid extraction from the C.sub.5 hydrocarbon fraction obtained by pyrolysis or dehydrogenation of hydrocarbons.
2. Description of the Prior Art
Isoprene is a starting material for production of poly-cis-1,4-isoprene useful as synthetic rubber, and recently it has been increasingly important. While various procedures have been proposed to produce isoprene, a method of separating and purifying isoprene contained usually in an amount of 10 to 20% in a C.sub.5 hydrocarbon fraction obtained by pyrolysis of hydrocarbons is commercially employed as one of the most economical procedures for the production of isoprene.
This C.sub.5 hydrocarbon fraction contains paraffins, olefins, diolefins, acetylenes and the like as well as isoprene. Furthermore, it contains a small amount of sulfur compounds resulting from organic sulfur compounds present as impurities in hydrocarbon feeds. Since many of these compounds have boiling points quite close that of isoprene or form azeotropic mixtures with isoprene, it is very difficult to separate and purify isoprene by conventional procedures. For the separation and purification of isoprene, extractive distillation and liquid-liquid extraction using polar solvents such as acetonitrile, dimethylformamide, N-methyl pyrrolidone and the like are known.
Even with these procedures, however, it is difficult to obtain high purity isoprene containing no polymerization inhibiting materials, suitable for use in the production of polyisoprene. Among the polymerization inhibiting materials, cyclopentadiene, acetylenes and sulfur are known to be especially harmful. therefore, it is quite important to minimize the concentration of these impurities to such an extent that they do not interfere with the polymerization of isoprene.
Various procedures have been proposed to remove acetylenes. For example, the azeotropic distillation process utilizing isopentane as described in Japanese Patent Publication No. 7664/1960, a method of adsorbing on a molecular sieve as described in Japanese Patent Publication No. 6202/1960, the extractive distillation process utilizing polar materials such as acetonitrile, dimethylformamide and the like as described in Japanese Patent Publication Nos. 26485/1968 and 41323/1972, the selective hydrogenation process as described in Japanese Patent Publication No. 22324/1970, a method of treating with alkali metals such as sodium and the like described in British Pat. No. 1070893 are known. These procedures, except for the azeoptropic distillation process utilizing isopentane, however, are not effective in removing 2-butyne. This azeoptropic distillation process also suffers from the disadvantage in that recycling and reuse of isopentane involve some difficulty, and it is not always an economical procedure.
On the other hand, for removal of dimethyl sulfide or other sulfur compounds having boiling points quite close to that of isoprene, several procedures have been proposed. For example, the azeotropic distillation process utilizing isopentane described in Japanese Patent Publication No. 7664/1960, and the method of treating with a solution of a silver salt in a polar solvent described in Japanese Patent Laid Open No. 83302/1975, and others, are known. The azeotropic distillation process utilizing isopentane, however, has disadvantages in that it is difficult to separate isopentane because it is liable to form an azeotropic mixture with dimethyl sulfide and in that recycling and reuse of isopentane involve some difficulty. Therefore, it is not always an economical procedure. The method of treating with a solution of a silver salt in a polar solvent has the defect that the silver salt is expensive. In addition, this procedure has the disadvantage that acetylenes usually present in crude isoprene together with dimethyl sulfide easily form silver acetylides, resulting in the formation of the quite desirable conditions for stable operation.